Missing: Voyager 1 yet to find the boundary line of the Solar System

Since today marks the 35th anniversary of the launch of the Voyager 1 spacecraft, it seems appropriate to commemorate the mission—and to note how it continues to provide data about the far edges of the Solar System.

Where exactly are those edges of the Solar System? According to theory, the boundary of the Solar System is marked by a region known as the heliopause, where the solar wind—particles streaming from the Sun—meets the plasma of interstellar space. In this region, beginning about 90 times the distance from Earth to the Sun, models predicted that the solar wind's particles would be deflected by the interstellar material, much as water is pushed aside by the bow of a ship.

However, new measurements provided by the venerable Voyager 1 probe have failed to find the expected flow, deepening the mystery of the boundary between our Solar System and interstellar space. This adds to an earlier surprise, when Voyager's instruments measured zero outward velocity in the solar wind, a measurement that has now held constant for over two years. In a Nature paper, Robert B. Decker, Stamatios M. Krimigis, Edmond C. Roelof, and Matthew E. Hill concluded that Voyager 1 is not actually close to the heliopause, despite expectations. The researchers further suggested that the models for interactions between the solar wind and interstellar plasma may require reevaluation.

The solar wind is a plasma: a mixture of electrons and (mostly) protons streaming out from the Sun. This flow varies a lot over time, based on the solar cycle, but its particles consistently move outward, or radially.

At some point, the wind will necessarily run into the material in the region between stars. This contains more plasma, along with non-ionized atoms and molecules, which are collectively known as the interstellar medium (ISM).

Diagram of the Solar System's boundary as predicted by theory, along with the unexpected "stagnation region" discovered by the Voyager 1 spacecraft.

By any reasonable theory, when the solar wind meets the ISM, there should be a transition. The usual model describes the solar wind carving out a region in the ISM known as the heliosphere, and the place where the solar wind is deflected by the pressure from the ISM is the aforementioned heliopause.

In April 2010, Voyager 1 reached a point about 113 AU from the Sun and saw the solar wind velocity began slowing down dramatically. (1 AU, or astronomical unit, is the average distance from Earth to the Sun, which is about 150 million kilometers.) This discovery led to speculation that the craft was reaching the heliopause.

If that was true, then the solar wind particles should be deflected in the transverse or meridional direction. Voyager's instruments happen to be good at measuring outward, or radial, flow velocity, so the spacecraft had to be reoriented five times over a 10-month period so that it could determine the meridional velocity of solar wind particles.

Two years of data revealed that the meridional velocity was 3±11 kilometers/second, or between -8 and +14 km/s. (Negative velocity in this case represents flow in the opposite direction expected by theory.) In other words, while the most likely velocity value is 3 km/s—within the instrumental limits, the flow is zero. That's certainly nowhere close to the predicted value of approximately 25 km/s.

Unlike the earlier "Pioneer anomaly," these data don't appear systematic: the five separate measurements were all consistent with zero meridional flow, but varied widely in most likely value.

These results are troublesome for several reasons. The thickness of the region where Voyager 1 has measured low radial velocity is now at least 7.5 AU (and counting). In that region, there has been very little fluctuation in the solar wind, even though we know the solar wind should vary strongly in time, thanks to the solar cycle. Additionally, combining the three components of the solar wind velocity shows the shape of the flow to be very different from what we expected.

The authors concluded that the heliopause either differs radically from theory, or Voyager is still not close to it. Both of these scenarios would require another look at the model for the solar wind-ISM interaction, since the current calculations don't explain the strange reduction in solar wind velocity or the direction of flow. It's possible the edge of the Solar System is farther out still—and might look very different from what we expected.

everytime i read a voyager article i can't help but think of the total mess at the end of star trek that was "Vyger." So lame!

I just want to say I might understand a quarter of this but it is still so cool. Pushing our understanding of the universe and exploring the cosmos, I'm all for efficiency with taxpayer dollars but unless the NASA administrator gets busted using funds to satisfy his/her bath salts habit, give these people all the money in the world.

Since the intergalactic winds are mostly theory and not measurement, I would not be surprised that reality does not match the models. Hopefully Voyager will hold together long enough to give some good data to the theorist to correct their models.

Since the intergalactic winds are mostly theory and not measurement, I would not be surprised that reality does not match the models. Hopefully Voyager will hold together long enough to give some good data to the theorist to correct their models.

What surprises me is that it's lasted this long! I mean, my last cell phone lasted two years before it fell apart, and NASA built a probe that's been around for several decades and is 16,800,000,000 kilometers* away! Really makes me wish NASA built cell phones LOL.

Seriously, though, the fact that we're still getting data from this thing is unbelievable. Talk about outliving your expected lifespan!

If the sun is continually shedding all this material, is it gradually losing mass? I think so.

Maybe I misunderstand it, but if the solar particles eventually are arrested, don't they just accumulate in a big spherical shell, the size of which is determined by a balance of the outward pressure of the solar wind against the inward pressure of the interstellar medium? The thickness being determined by all those particles streaming outward from the sun...

Outward velocity dropping to zero would then make sense as the particles are just accumulating after becoming part of this boundary layer.

The motion on the other two axes might be explained as due to rotation of the entire system (very slow but, at those distances, seemingly high compared to the units we are accoustomed to) or more likely eddies or turbulence in this boundary layer due to the non static nature of nature.

(If any of this is novel or correct, I'd like to know so I could seek attribution for the ideas. How does one do that anyway?)

Since the intergalactic winds are mostly theory and not measurement, I would not be surprised that reality does not match the models. Hopefully Voyager will hold together long enough to give some good data to the theorist to correct their models.

What surprises me is that it's lasted this long! I mean, my last cell phone lasted two years before it fell apart, and NASA built a probe that's been around for several decades and is 16,800,000,000 kilometers* away! Really makes me wish NASA built cell phones LOL.

Seriously, though, the fact that we're still getting data from this thing is unbelievable. Talk about outliving your expected lifespan!

* - I don't know why I typed miles the first time around

That would be great, except the phones would cost a quarter million dollars each. On the upside they would last forever and the coverage would be outstanding!

If the sun is continually shedding all this material, is it gradually losing mass?

Strictly, yes. Each second, more than four million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation. At this rate, the Sun has so far converted around 100 Earth-masses of matter into energy. Luckily, the Sun is about 333,000 Earth-masses, so losing 100 doesn't really hurt it.

If the sun is continually shedding all this material, is it gradually losing mass? I think so.

Maybe I misunderstand it, but if the solar particles eventually are arrested, don't they just accumulate in a big spherical shell, the size of which is determined by a balance of the outward pressure of the solar wind against the inward pressure of the interstellar medium? The thickness being determined by all those particles streaming outward from the sun...

Outward velocity dropping to zero would then make sense as the particles are just accumulating after becoming part of this boundary layer.

The motion on the other two axes might be explained as due to rotation of the entire system (very slow but, at those distances, seemingly high compared to the units we are accoustomed to) or more likely eddies or turbulence in this boundary layer due to the non static nature of nature.

(If any of this is novel or correct, I'd like to know so I could seek attribution for the ideas. How does one do that anyway?)

Yes, the sun is continually losing mass but mostly because it's converting a boatload of hydrogen into helium. The lost mass is converted into energy and is why the sun "shines." I'm not certain if the mass exchange of ejected particles vs. captured dust is positive or negative.

In rarefied flow conditions (where the spacing between particles is huge) you won't get an accumulation. That's more akin to a shock in front of a supersonic craft. However, fly that same craft high enough that the atmosphere is thin and shock waves disappear. Instead, you end up with one stream of really fast moving particles passing through a field of stagnant particles and the slow rate of collisions means that there's a very large volume over which the fast-moving particles slow down.

I suspect we'll find that the flow between our space and interstellar space has geometrical complexity to it - like a Earth's magnetosphere. Heck, we're sending up craft just to understand the van Allen belts and we have a LOT more measurements of the neighborhood in that case. I'm almost shocked that anyone expected the actual heliosphere boundary to be exactly what was predicted. On average? sure. At one specific point? No way.

Since the intergalactic winds are mostly theory and not measurement, I would not be surprised that reality does not match the models. Hopefully Voyager will hold together long enough to give some good data to the theorist to correct their models.

What surprises me is that it's lasted this long! I mean, my last cell phone lasted two years before it fell apart, and NASA built a probe that's been around for several decades and is 16,800,000,000 kilometers* away! Really makes me wish NASA built cell phones LOL.

Seriously, though, the fact that we're still getting data from this thing is unbelievable. Talk about outliving your expected lifespan!

* - I don't know why I typed miles the first time around

It is travelling in space, not being dipped into toilets here on Earth.

What surprises me is that it's lasted this long! I mean, my last cell phone lasted two years before it fell apart, and NASA built a probe that's been around for several decades and is 16,800,000,000 kilometers* away!

For a moment I thought Voyager was speeding away as fast as the deficit was rising, but then I realized it's not quite that far, by a factor of 1000.

Unrelated, it's not really surprising that Voyager's instruments are not reading what is expected. All science is an educated guess based on observation until someone makes the next better sounding guess.

We should be teaching that the Solar System is merely a theory, and that alternatives should be introduced into the classroom!

/sarcasm

I laughed. Or smirked, anyway. But then I think it's generally good to have reminders that some people think "gaps" in scientific knowledge (e.g. "missing" solar wind here) are somehow strikes against the effectiveness of the scientific process. They're laughably wrong.

robert.walter wrote:

If the sun is continually shedding all this material, is it gradually losing mass?

Maybe I misunderstand it, but if the solar particles eventually are arrested, don't they just accumulate in a big spherical shell, the size of which is determined by a balance of the outward pressure of the solar wind against the inward pressure of the interstellar medium? The thickness being determined by all those particles streaming outward from the sun...

Outward velocity dropping to zero would then make sense as the particles are just accumulating after becoming part of this boundary layer.

The motion on the other two axes might be explained as due to rotation of the entire system (very slow but, at those distances, seemingly high compared to the units we are accoustomed to) or more likely eddies or turbulence in this boundary layer due to the non static nature of nature.

That seems like a reasonable hypothesis (I'm not an astronomer/physicist though). I think the problem is that we can't know if that's true until Voyager starts detecting a consistent particle velocity again (though perhaps with a different variety or ratio of particles), which would suggest it has passed beyond the heliopause and into the ISM.

Since the intergalactic winds are mostly theory and not measurement, I would not be surprised that reality does not match the models. Hopefully Voyager will hold together long enough to give some good data to the theorist to correct their models.

What surprises me is that it's lasted this long! I mean, my last cell phone lasted two years before it fell apart, and NASA built a probe that's been around for several decades and is 16,800,000,000 kilometers* away! Really makes me wish NASA built cell phones LOL.

Seriously, though, the fact that we're still getting data from this thing is unbelievable. Talk about outliving your expected lifespan!

* - I don't know why I typed miles the first time around

If NASA built cellphones there would be 1 production unit, 5 functional prototypes, 15 more for parts, they would be built entirely out of titanium and myriad rare-earth metals, would be four times their current size, and it would take 30+ scientists and 100+ engineers 10 years and 200 billion to produce it. However, it would last forever and essentially be invulnerable to unimaginable conditions. It might even do facebook.

But if you want a phone that meets most of that criteria and doesn't cost a nation's debt, I have an old Nokia 3310 in my closet that will still fire up despite having taken enough abuse to utterly destroy any phone on the market today.

If NASA built cellphones there would be 1 production unit, 5 functional prototypes, 15 more for parts, they would be built entirely out of titanium and myriad rare-earth metals, would be four times their current size, and it would take 30+ scientists and 100+ engineers 10 years and 200 billion to produce it. However, it would last forever and essentially be invulnerable to unimaginable conditions. It might even do facebook.

They would, however, manage to do this with one half of one percent of the US budget. Imagine what they could accomplish if we doubled their budget and gave them a full penny out of every tax dollar.

What surprises me is that it's lasted this long! I mean, my last cell phone lasted two years before it fell apart, and NASA built a probe that's been around for several decades and is 16,800,000,000 kilometers* away! Really makes me wish NASA built cell phones LOL.

I'm sure you could find a ruggedized Nokia 1011 that still works on a 1G network somewhere.;-pVoyager 1 has bulky space & radiation-hardened redundant computer systems, at ~80kips with a total of 32kB NVRAM. Durability has many costs.

The answer, as usual, is "science". And more specifically, as is often the case in cosmology, "light".

Makes sense, but from the link, they seem to mostly be talking about "clouds" - That part I can understand. You find something different, you can measure it. But how do you measure the "non-clouded" part? As I think someone else mentioned, you do have to make some assumptions about the boundary you're measuring through, right? And if that turns out to be off (which so far seems like it might be the case based on the readings from Voyager), what is that going to do to the models?

We should be teaching that the Solar System is merely a theory, and that alternatives should be introduced into the classroom!

/sarcasm

I laughed. Or smirked, anyway. But then I think it's generally good to have reminders that some people think "gaps" in scientific knowledge (e.g. "missing" solar wind here) are somehow strikes against the effectiveness of the scientific process. They're laughably wrong.

What I find unfortunate is that the extremists that drop all of science just because there are gaps in knowledge make those, like myself, who are religious look ridiculous. I love the scientific process. It has given amazing results. My only issue with science is not with science itself. It's with the people involved.

After reading Einstein by Isaacson, specifically where he had to defend his claims before the (paraphrased) Newtonian Court, I felt like he was being held on trial for violating some deeply religious beliefs. IMHO it seems like each generation, religious or scientific, like to believe that they are on the verge of discovering it all. Just seems small minded to me to think that even with all the amazing discoveries we've made, that it's anything more than the tip of the ice berg of scientific knowledge still left undiscovered.

On topic, what I'd give for a high quality color picture backwards. I wonder how impossibly tiny the sun appears from where Voyager is now.

I can't imagine it would look much different, superficially, from any random star field photo that you can find. I mean, you'd be able to point to a brightish dot and say "that's the sun" or maybe pick out a planet or two (or maybe not?), but it would just be a bunch of stars, basically.

Has the solar wind velocity dropped to zero? I understood from the NASA article that it's been stable for the past 2 years at half of what it was for the 5 previous years, not null.

Even if Voyager 1 had reached the ISM, wouldn't it be a space of much lower density and activity compared to the space where solar wind still flows? If the solar wind made it so far, I would be expecting it to continue unabated for many billion kilometers more until the last particles eventually collide with others, in a kind of a smooth decline, not an abrupt transition like the one you can have for the solar system's gravity field.Or where we expecting the "turbulent" zones of the ISM to be present right at the edge of our solar system?

Regardless, Voyager 1 is a fantastic source of knowledge and seems just as limitless as our imagination.

If NASA built cellphones there would be 1 production unit, 5 functional prototypes, 15 more for parts, they would be built entirely out of titanium and myriad rare-earth metals, would be four times their current size, and it would take 30+ scientists and 100+ engineers 10 years and 200 billion to produce it. However, it would last forever and essentially be invulnerable to unimaginable conditions. It might even do facebook.

They would, however, manage to do this with one half of one percent of the US budget. Imagine what they could accomplish if we doubled their budget and gave them a full penny out of every tax dollar.